You've something wrong.
I'm copying 3 TB right now. Speed is indeed only about 100Mbps, but my CPU load is reported as: CPU Load Average (1, 5, 15 mins) 2.31, 2.13, 2.09
If you hit 50 or 25%, something is wrong..
I heard with ARM routers you get 400Mbps, but who has ARM routers...